The present invention relates generally to optical amplifiers used in fiber-optics for telecommunications, cable television and other fiber-optic applications. More particularly, the invention relates to an optical fiber amplifier utilizing Raman amplification in fibers and employing an optical circulator cavity and/or chirped Bragg gratings.
With the large increase in usage of the internet, world-wide web, and other computer communications applications, the demand for communication capacity (often referred to as xe2x80x9cbandwidthxe2x80x9d)has skyrocketed. Because of the high cost of installing new fibers and obtaining right-of-way, there is a large incentive to exploit to the fullest extent possible the bandwidth available in the embedded base of optical fibers.
The bandwidth of optical fibers is based on two key technologies: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM refers to different wavelengths of light traveling in parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth by using multiple xe2x80x9clanesxe2x80x9d of fiber xe2x80x9chighway,xe2x80x9d WDM would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for obtaining use of the full fiber bandwidth.
There are two main low-loss telecommunications windows in optical fibers at wavelengths of 1.3 xcexcm and 1.55 xcexcm respectively. While Erbium-doped fiber amplifiers (EDFAs) have become the workhorse in the optical amplifier field, they only operate in the 1.55 xcexcm window. Raman amplifiers have the advantage that they can operate in both optical communication windows, and, in fact, over the entire transparency window of optical fibers. Moreover, Raman gain increases system reliability since there is no excess loss in the absence of pump power. Raman based amplifiers are fully compatible with fiber systems since they are all-fiber devices.
As with any amplifier, noise is a consideration in deciding whether to use a Raman amplifier. The theoretical noise figure contribution from signal-spontaneous beating for Raman amplifiers has been shown to be 3 dB. However, system tests of Raman amplifiers have uncovered other sources of noise that generally are not important in EDFAs (cf. A. E. White and S. G. Grubb, xe2x80x9cOptical Fiber Components and Devices,xe2x80x9d Ch. 7 in Optical Fiber Telecommunications IIIB, eds. I. P Kaminow and T. L. Koch. Academic Press, 1997). The first source of noise is from the coupling of intensity fluctuations from the pump light to the signal. The fundamental cause of this noise is the lack of a long upper-state lifetime to buffer the Raman gain from fluctuations in the pump intensity. It has been shown that when a counter-propagating amplifier geometry is used, the transit time of the amplifier can be used to average gain fluctuations due to the pump.
Second, double Rayleigh scattering can also give significant contributions to the noise figure of Raman amplifiers because of the long lengths of fiber used. Double-Rayleigh scattering corresponds to interaction between the signal and pump, where the signal is reflected by Rayleigh scattering in the backwards direction, but then it is scattered again in the forward direction by a second Rayleigh scattering. The scattered signal then interferes with the original signal, giving rise to noise degradation. The Rayleigh scattering is enhanced in long fiber lengths and in fibers with small core sizes and high core-cladding index differences, such as fibers with enhanced Raman cross-section. Double Rayleigh scattering has been observed in the past. The prior art solution to control the noise figure of the amplifier is to limit the fiber lengths used and construct multistage amplifiers.
Several Raman laser and amplifier cavity designs exist as prior-art, but they are not very appropriate for broadband amplification of WDM systems. S. G. Grubb and A. J. Stentz have described in Laser Focus World, pp. 127-134, (1996) a linear cavity that uses a series of gratings to define the end mirrors. However, the bandwidth of the gratings is sufficiently restrictive that the cavity operates over only about 2 nm. This is inadequate for WDM applications. As an improvement, Grubb, et al. in U.S. Pat. No. 5,623,508 also describe a ring cavity that uses an intra-cavity isolator to reduce double Rayleigh scattering and uses a counter-propagating pump to avoid pump fluctuations from coupling to the signal channel. The ring cavity design, however, is substantially more complicated, and since it also employs gratings it is also narrow band. Rather than using gratings, Chernikov, et al. (Electronics Letters, Vol. 31, pp. 472-473, March 1995), use wavelength selective couplers in their Raman cavity design. By using these broader band devices, they achieve a bandwidth of 6-10 nm. However, the couplers may be tricky to manufacture, and there are no means in the cavity for rejection or dampening of the double Rayleigh scattering.
A broadband Raman amplifier which is suitable for WDM systems has been experimentally demonstrated using a pump source consisting of the combined output of 8 high-powered semiconductor diode lasers (Optical Amplifier Conference 1988, ED3-2) which were only a single Raman order away from the desired peak gain. Since no Raman wavelength shifting of the pump was required, the power at each pump wavelength could be adjusted directly by adjusting the drive current of each diode laser. This allowed the gain spectrum of the amplifier to be precisely controlled. While a Raman amplifier using a combination of many laser diodes as the pump source offers a straightforward way of producing a Raman amplifier with a large optical bandwidth, there are some drawbacks. One such drawback is the inherent complexity of such an optical assembly, especially if very high pump powers are required. To double the output power of the pump requires at least twice as many laser diodes, polarization beam combiners, and WDMs be used. Another drawback is that the failure of a single pump laser would make it difficult to produce a flat gain spectrum by adjusting the output powers of the remaining devices, making reliable operation more difficult to achieve.
The present invention provides a structure for optical amplification of signals with counter-propagation of signal and pump beam and wavelength control while permitting broad bandwidth. The broadband optical amplifier of the invention combines optical amplification with a circulator loop cavity and/or chirped Bragg gratings to achieve bandwidth performance improvements that neither technology by itself has heretofore been able to deliver. The beneficial properties of the circulator loop cavity and/or chirped Bragg gratings can also combined with noise dampening property of the Sagnac Raman cavity, as described in U.S. Pat. No. 5,778,014.
In one embodiment of the circulator loop cavity, a counter-propagating pump beam amplifies the signal. Two ends of the circulator are connected with a gain medium. The signal travels through the gain medium in the opposite direction, and wavelength selective couplers are used to introduce and remove the signal. The intensity fluctuations of the pump beam is dampened by combining the circulator loop cavity with a Sagnac Raman amplifier, which has the property of reflecting the common mode signal while partially rejecting the difference mode noise.
The present invention also relates to wavelength control in the Raman amplifier through use of chirped Bragg gratings. A series of chirped Bragg gratings with reflection bands centered around the various Raman orders is attached to a third port of the optical circulator. The bandwidth is adjusted to permit for expanded bandwidth in each subsequent Raman order. In addition, gain flattening can be achieved by apodizing the gratings or by introducing filters with reflection coefficients opposite to the Raman gain spectrum.
Means of reducing the noise from double-Rayleigh scattering in a circulator loop cavity are also described. The use of multiple-stages of gain fiber connected with an isolator reduces the deleterious effects of double-Rayleigh scattering. The isolation of the circulator itself reduces the reflections, and the counter-propagating aspect of the signal and control can be achieved by adding a WDM coupler and a separate path for the pump orders. The wavelength control is implemented using chirped Bragg gratings by introducing an additional circulator into the separate pump path.
The chirped Bragg gratings are also used advantageously in other cavity designs to broaden the bandwidth. The invention discloses the use of chirped Bragg gratings in Sagnac Raman cavities or linear Fabry-Perot cavities to permit bandwidth expansion. Additionally, the gain bandwidth is forced to grow during each cascade Raman order. A broadband or multi-mode pump should be employed. The bandwidth increases during each cascade order due to the convolution with the xcx9c20 nm Raman gain bandwidth. Further bandwidth expansion is accomplished by placing at least one of the intermediate Raman orders in close proximity to the fiber zero dispersion wavelength, so as to phase-match four-wave mixing or parametric amplification.
Finally, methods for enhancing the disclosed embodiments are discussed. Several efficiency improving cavity variations are detailed and an alternative method for enhancing the bandwidth of a Raman amplifier by producing multiple discrete pump wavelengths from a single-wavelength pump is disclosed.